Recently, with practical application of a GaN type laser diode, development of an optical disc information system which performs high density recording to an optical disc by using a light having a short wave length in a range from 380 nm to 430 nm is active. In these optical disc systems, reading from an optical disc is performed at a low power and writing to an optical disc is performed at a high output. A GaN-based laser diode having a single striped structure has been used as a light source, and both of reading and writing are performed by changing an output of the laser diode.
However, in a case where reading and writing are performed with a typical GaN-based laser diode having a single striped structure, there has been the following problems when data are transferred at high speed with a high reading/writing ratio (hereinafter referred to as RW ratio) of laser output. In an optical information disc system, it is necessary for reading to maintain a value of relative intensity noise (hereinafter referred to as RIN value) low, and for writing to generate a high output with a single transverse mode for a longer operating life. However, it is difficult to maintain the RIN value at a level necessary for reading by simply decreasing output at reading in order to increase the RW ratio, because the RIN value generally decreases as output decreases. On the other hand, it is also difficult to maintain a single transverse mode and a long operating life of the laser element by simply enhancing output at writing in order to increase the RW ratio, because the single transverse mode and the operating life decreases as the output rises.
Also, in design of a laser having a striped structure, a high maximum output cannot be obtained when it is designed to decrease a lower RIN value at a low output. On the contrary, in a design to obtain a high maximum output, the RIN value at a low power output increases, so that it is difficult to satisfy both of these properties with a single laser striped structure.
For this reason, it is difficult to transfer data at high speed with a high RW ratio at high speed while maintaining a necessary RIN value and a longer operating life, when a GaN-based laser diode with a typical single striped structure is used to perform both reading and writing.
It is therefore an object of the present invention is to provide a GaN-based semiconductor laser device which can transfer data with a high RW ratio at high speed while maintaining a necessary RIN value for an optical disc information system and a longer operating life.